Typical wireless mesh networks use a single radio for the backhaul or relay function where packets are moved through the mesh from node to node. This causes a significant bandwidth limitation since a single radio cannot send and receive at the same time. Adding relay radios at individual mesh nodes can enable a mesh node to simultaneously send and receive packets, thereby increasing the overall rate of bandwidth propagation through the mesh node. The simplest form of prior art mesh network is the ad hoc mesh network shown in FIG. 1(a), where each mesh node 101 contains a relay radio 102. This is the most elemental form of wireless mesh network and originated in the military. It was characteristic of these networks that all mesh nodes have a single radio and all radios operate on the same channel or frequency.
Note that in this specification, the term “channel” is most often used to mean a specific RF frequency or band of frequencies. However, the term “channel” is to be understood in a generalized sense as designating a method of isolating one data transmission from others such that they do not interfere. While this differentiation or isolation may be accomplished by utilizing different frequencies, it may also be accomplished by choosing different RF wave polarizations or in the case of a TDMA scheme, it may refer to different time slots in a time division scheme. For CDMA systems, isolation of transmissions may result from having different spreading codes. Regardless, channelization is a method for making efficient use of available spectrum and preventing interference between different transmissions that otherwise might interfere with each other.
One evolution of the early ad hoc mesh network form is shown in FIG. 1(b) where relay radio 103 is capable not only of transferring packets to adjacent nodes, but is also capable of operating as an access point (AP) as well, providing service (typically WiFi) to client devices such as laptop computers, wireless PDAs, and WiFi VoIP phones.
The architecture of FIG. 1(b) suffers from performance limitations since the single radio must not only relay packets, but also service numerous client radios 104 at each node. Thus, another evolution was developed as shown in FIG. 1(c), where each mesh node has a separate service or AP radio 105 in addition to relay radio 106. This allows client devices 107 to communicate with service radio 105 on a different channel or frequency than relay radio 106, thereby reducing interference effects within the mesh and increasing performance.
A more recent evolution of mesh architectures is shown in FIG. 1(d) where relay radios 108 and 109 are used at each mesh node along with a separate service radio 110. Here, packets can be received on relay radio 108 while simultaneously being transmitted on relay radio 109, and vice versa, thereby increasing performance due to both the simultaneous operation of both radios, as well as the fact that radios 108 and 109 typically operate on different channels, thereby further reducing interference effects in the mesh. It is also known to add radios to the architecture shown in FIG. 1(d) such that there would be two relay radios for uplink replacing relay radio 108, and two relay radios for downlink replacing relay radio 109. This addition effectively doubles the bandwidth and enables full-duplex (simultaneous uplink and downlink) operation, however a specific packet stream will propagate through only one of a pair of uplink or downlink radios. Thus, the maximum performance of such a link between two nodes will only be realized in situations where traffic loading is high. The absolute performance of a single stream of packets will not be increased beyond what a single link could deliver.
While FIG. 1 shows the architectures for various prior art mesh networks in a one-dimensional form for sake of simplicity, FIG. 2 elaborates on the architecture of FIG. 1(d) showing a two-dimensional view. In the 3-radio mesh of FIG. 2, also known as a “structured” mesh, a tree-like structure is formed emanating from a root node 201 which connects directly to a wired network 202. This wired network can, in turn, connect to the Internet or alternatively, it may connect simply to a server. In the case of a public safety network, the wired network will often connect to the Command and Control center. It is characteristic of this type of mesh that, at every hop, packets being relayed travel on a different channel from the previous hop. Thus RF transmissions, 202, 203, and 204 which connect mesh node 201a with mesh nodes 205, 206, and 207, operate on three different channels or frequencies as shown by the different styles of dotted line. In this type of mesh network, the mesh control software on each node has a significant challenge in assigning the various available channels throughout the mesh such that interference effects are minimized, and the mesh functions properly. Some mesh network vendors rely on customers to manually assign channels as the units are being installed. Other mesh vendors have developed very elaborate dynamic channel assignment software programs, which perform this function automatically. Either way, having a mesh network where channels change from hop to hop is complicated and difficult to deal with. In the case of a public safety mesh with mobile nodes (for vehicles and individual First Responders on foot), a further problem arises with this form of mesh. For instance, if a group of first responders each carrying a mesh node become isolated from the backhaul connection to the server (Command and Control), the tree-like structure of FIG. 2 may become compromised since there is no longer a defined root for the tree. It is important for isolated groups of first responders, with nodes that are vehicle mounted, man-carried, or both, to continue communicating amongst themselves when isolated until the connection to Command and Control is restored.
FIG. 3 shows example channel configurations in a WLAN Mesh from section 4.2.3 of IEEE 802.11-06/0328r0, the Combined Proposal for the ESS Mesh Standard (published in March 2006). It should be noted that the publication referenced here post dates the filing of U.S. Provisional Application Ser. No. 60/756,794 to which the present application claims priority. However, in the event that this information had been published in previous submittals at prior IEEE standards meetings, and also for purposes of clarity, the information in this publication is being described herein. FIG. 3(a) shows a simple ad hoc mesh, while FIG. 3(b) shows two ad hoc meshes, 301 and 302, which are bridged by central mesh node 303 having two radios. FIG. 3(c) shows a number of mesh nodes, each having two radios for packet relay, which for the most part are being utilized in a manner similar to the “structured” mesh of FIG. 2. FIG. 3(c) also demonstrates the concept of nodes with 2-radio relays being used to bridge between one sub-mesh and another. This referenced proposal for a new mesh standard also discusses the concept of Unified Channel Graphs or UCGs. In FIGS. 3(d) and 3(e), notice that FIGS. 3(b) and 3(c) are replicated with superimposed circles 304 indicating nodes which communicate with each other on a particular channel. Essentially FIG. 3(e) demonstrates a number of sub-meshes which are bridged by mesh nodes, each bridging node containing two relay radios. One can easily imagine the challenge in assigning channels to the network demonstrated in FIGS. 3(c) and 3(e). Also, when connections between nodes must change because of a node failure, temporary disturbances to the mesh (moving obstacles or radar interference), node movement, or QOS considerations, there can be a ripple effect of changing channels causing even greater complexity.
FIG. 4 shows the architecture for the only mesh network solution that currently supports both public safety and public access, and is being sold by Motorola. Here, there are two completely separate mesh systems embodied in the same enclosure 401. Each enclosure has two radios 402 for public safety and two radios 403 for public access. Each of these separate meshes functions as a “1+1” mesh as demonstrated in FIG. 1(c) by radio elements 105 and 106. This vendor has chosen to make the public access radios utilize 2.4 GHz for both relay and service, with 4.9 GHz being utilized for the public service radios (relay and service). Each of these meshes is separate from the other with no interaction. In particular, packet traffic on the 4.9 GHz mesh may only be used for public service as governed by law—public access traffic may never utilized 4.9 GHz. Thus, this prior art solution addresses the problem that it is desirable to reduce the number of mesh unit enclosures that must be mounted at strategic locations to cover a metropolitan area. However, the solution does not integrate any additional functionality beyond what is shown in FIG. 4, and from a performance standpoint, each of the two individual mesh networks embodied here will have the performance restrictions of other prior art mesh architectures constructed according to FIG. 1(c).
It would therefore be desirable to have a wireless mesh network architecture with the performance characteristics provided by a 2-radio relay, without the complexity of managing multiple and dynamically changeable channels, which can change from hop-to-hop.
The majority of mesh nodes being installed today use omnidirectional antennas for the relay or backhaul function to transfer packets between mesh nodes. While some mesh vendors claim to have installed mesh networks in hundreds of cities, all but a few of these are suburban towns, not large cities with tall buildings. In fact, none of the mesh systems offered today have been designed to handle the problems encountered in the depths of larger cities where high rise buildings create a “concrete canyon” effect. When today's mesh nodes are deployed in such situations, much of the energy radiated from their omni-directional antennas is reflected and/or wasted. As will be shown in FIGS. 11 and 12, in such circumstances most of the energy radiated from a relay radio's omnidirectional antenna is directed at buildings, rather than down the street corridor to where other mesh nodes are located. Here, directional or sector antennas can offer significant advantages. Throughout this specification, directional and sector antennas are often used interchangeably. This is because they sometimes are interchangeable when one desires to focus the transmitted RF radiation, depending on just how narrow a beam is desired. In one sense, any antenna that is not “omnidirectional” can be considered “directional”. However, among RF engineers, there is often a distinction between sector and directional antennas, as they differ to some extent. A sectoral or sector antenna has a horizontal beam angle that is measured in substantial portions of 180 degrees, most frequently, 90 degrees. They are often available with horizontal beam angles as small as 30 degrees, and one can think of them as covering a piece of the “360 degree pie”, hence the term “sector”. To focus the RF energy even more, a variety of types of “directional” antennas are available, usually with significantly higher gains. Directional antennas come in a variety of configurations referred to as “dish”, “panel”, “patch”, or “reflector grid”, to name a few. A 32 dBi dish antenna, for instance, would have both horizontal and vertical beam widths of 5 degrees, not something one would think of as covering a “piece of a pie” as with sector antennas.
Other factors involved in mesh node and mesh architecture design involve both the transmit power and cost of radio cards. The cost of radio cards for wireless networks is becoming increasingly lower, and although many of these have relatively low power, when combined with directional or sector antennas the EIRP (total transmitted power output from the antenna) can be more than acceptable, especially if utilized in a city deployment where the transmit energy can be focused in order to propagate between buildings, rather than wasted by transmitting into buildings.